Prediction of Process-Induced Residual Stresses in Thermoplastic Composites

A model to predict the macroscopic in-plane residual stress state of semi crystalline thermoplastic composite laminates induced by process cooling is presented. Heat transfer during processing is based upon an incremental transient formulation that consists of a finite difference heat transfer analysis coupled to the crystallization kinetics. Micromechanics is used to evaluate the instantaneous spatial variation of mechanical prop erties as a function of temperature and degree of crystallinity. Residual stresses are based upon an incremental laminate theory that includes temperature gradients, shrinkage due to crystallization and thermal contraction. Temperature dependent relaxation times are used to model first order viscoelastic effects. A parametric study is conducted to explore the sensitivity of residual stresses to process history. Input parameters varied include surface temperature history (cooling rate), the amount of shrinkage caused by crystallization and the relaxation time at the reference temperature. The model predictions were in good agreement with experimental residual stress measurements for unidirectional graphite (AS4) reinforced polyetheretherketone (PEEK) laminates.